Hydrofluorocarbons (HFCs) and in particular hydrofluoroolefins (HFOs), such as 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf), are compounds known for their properties of refrigerants and heat-exchange fluids, extinguishers, propellants, foaming agents, blowing agents, gaseous dielectrics, polymerization medium or monomer, support fluids, agents for abrasives, drying agents and fluids for energy production units. Unlike CFCs and HCFCs, which are potentially dangerous to the ozone layer, HFOs do not comprise chlorine and thus do not present a problem for the ozone layer.
1,2,3,3,3-Pentafluoropropene (HFO-1225ye) is a synthetic intermediate in the manufacture of 2,3,3,3-tetrafluoro-1-propene (FIFO-1234yf).
The majority of the processes for the manufacture of hydrofluoroolefins involve a dehydrohalogenation reaction. Thus, the document WO 03/027051 describes a process for the manufacture of fluoroolefins of formula CF3CY═CXnHp, in which X and Y each represent a hydrogen atom or a halogen atom chosen from fluorine, chlorine, bromine or iodine and n and p are integers and can independently take the value zero, 1 or 2, provided that (n+p)=2, which comprises bringing a compound of formula CF3C(R1aR2b)C(R3cR4d), with R1, R2, R3 and R4 independently representing a hydrogen atom or a halogen atom chosen from fluorine, chlorine, bromine or iodine, provided that at least one of R1, R2, R3 and R4 is a halogen atom and that at least one hydrogen atom and one halogen atom are situated on adjacent carbon atoms, a and b being able independently to take the value zero, 1 or 2, provided that (a+b)=2, and c and d being able independently to take the value zero, 1, 2 or 3, provided that (c+d)=3, into contact with at least one alkali metal hydroxide in the presence of a phase transfer catalyst.
This document teaches, in Example 2, that, in the absence of a phase transfer catalyst, there is no reaction when 1,1,1,3,3-pentafluoropropane (HFC-245fa) is brought into contact with a 50% by weight aqueous potassium hydroxide (KOH) solution at ambient temperature and under pressure for 24 hours.
In addition, this document teaches a reaction temperature of between −20° C. and 80° C.
The document WO 2008/075017 illustrates the dehydrofluorination reaction of 1,1,1,2,3,3-hexafluoropropane (HFC-236ea) to give 1,2,3,3,3-pentafluoropropene (HFO-1225ye) at 150° C. in the presence of a 50% by weight aqueous KOH solution. In the absence of a phase transfer catalyst, the conversion after 3 and a half hours is 57.8% and the selectivity for HFO-1225ye is 52.4% (Test 1). In the presence of a phase transfer catalyst, this conversion is reached after only 2.5 hours and the selectivity is virtually unchanged (Test 4). As indicated in Table 2 of this document, it is necessary to use an organic solvent in order to increase the selectivity for HFO-1225ye.
WO 2007/056194 describes the preparation of HFO-1234yf by dehydrofluorination of 1,1,1,2,3-pentafluoropropane (HFC-245eb) either with an aqueous KOH solution or in the gas phase in the presence of a catalyst, in particular over a catalyst based on nickel, carbon or a combination of these.
The document Knunyants et al., Journal of the USSR Academy of Sciences, Chemistry Department, “Fluoroolefin Reactions”, Report 13, “Catalytic Hydrogenation of Perfluoroolefins”, 1960, clearly describes various chemical reactions on fluorinated compounds. This document describes the dehydrofluorination of 1,1,1,2,3,3-hexafluoropropane (236ea) by passing through a suspension of KOH powder in dibutyl ether, to produce 1,2,3,3,3-pentafluoro-1-propene (HFO-1225ye) with a yield of only 60%. This document also describes the dehydrofluorination of 1,1,1,2,3-pentafluoropropane (HFC-245eb) to give 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf) by passing into a suspension of KOH powder in dibutyl ether with a yield of only 70%.
Furthermore, FIG. 2 on page 51 of Part 2 of the nouveau traité de chimie minérale [New Treatise on Inorganic Chemistry] by P. Pascal, Ed. 1963, shows the appearance of the liquid/solid equilibria of the water and potassium hydroxide system and the measurements are collated in the Table on page 52.
The dehydrofluorination reactions such as described above result, besides the desired hydrofluoroolefin compound, in the formation of water and potassium fluoride. Furthermore, the implementation of such a reaction in continuous mode is not easy on an industrial scale since at least three phases (gas, liquid and solid) are involved.